<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
            margin: 0;
            width: 100%;
            display: flex;
        }
        footer {
            position: fixed;
            bottom: 0;
            width: 100%;
            text-align: center;
            font-size: 30px;
            display: none;
        }
        main {
            height: 100vh;
            background-color: #ccc;
            flex: 1;
        }
        aside {
            height: 100vh;
            width: 200px;
            background-color: #aaa;
        }
        @media screen and (max-width: 800px) {
            aside {
                display: none;
            }
            footer {
                display: block;
            }
        }
    </style>
</head>
<body>
    <main></main>
    <aside></aside>
    <footer>hello</footer>
</body>
</html>